The use of antiwear agents in lubricating compositions is known. The antiwear agent may be either ash-containing or ashless lubricating compositions. The ashless and ash-containing antiwear agents may be employed in a wide variety of mechanical devices including hydraulic systems, driveline systems and internal combustion engines.
Typically, additives that tend to decompose are often ash-containing additives that contain a metal and often phosphorus (such as zinc dialkyldithiophosphate). Furthermore, as ash-containing antiwear agents decompose, divalent metals such as zinc, calcium or magnesium, are released into the lubricating composition. Once released, the metal may have a detrimental impact on filters (by causing filter plugging), poison catalysts or other after-treatment devices, cause other pollution, or increase sludge or sediment. In addition, ashless antiwear agents are believed to have at least one of limited oxidative stability, limited thermal stability, and produce unacceptable levels of sludge.
In addition, ashless and ash-containing antiwear agents are believed to be consumed relatively quickly compared with a general lubricant change cycle, consequently increasing wear.
European Patent Application EP 0 821 053 A2, International Applications WO 00/11122 and WO 04/113479, US Patent Application 2002/0010103, and U.S. Pat. No. 5,922,657, all disclose ashless antiwear packages for hydraulic fluids.
US Application US 2006/0189490 (equivalent to WO 04/087850) and WO 07/025,837 both disclose lubricating composition containing block copolymers prepared from RAFT (Reversible Addition Fragmentation Transfer) or ATRP (Atom Transfer Radical Polymerisation) polymerisation processes.
International Applications WO 06/047393, WO 06/047398, WO 07/127,615 (U.S. 60/745,422), WO 07/127,660 (U.S. 60/745,420), WO 07/127,663 (U.S. 60/745,417), and WO 07/127,661 (U.S. 60/745,425) all disclose RAFT polymers for lubricants. The RAFT polymers provide thickening to a lubricant.
International Application WO 96/23012 discloses star-branched polymers prepared from acrylic or methacrylic monomers. The polymers have a core or nucleus derived from acrylate or methacrylate esters of polyols. Further the polymers have molecular weights and other physical characteristics that make them useful for lubricating oil compositions. The star-branched polymers disclosed are prepared by anionic polymerisation techniques.
The star polymers of EP 979 834 require from 5 to 10 weight percent of a C16 to C30 alkyl (meth)acrylate and from 5 to 15 weight percent of butyl methacrylate. A viscosity index improver with a C16 to C30 alkyl (meth)acrylate monomer present at 5 weight percent or more has reduced low temperature viscosity performance because the polymer has a waxy texture.
U.S. Pat. No. 5,070,131 discloses gear oil compositions having improved shear stability index essentially consisting of gear oil, a viscosity index improver comprising a hydrogenated star polymer comprising at least four arms, the arms comprising, before hydrogenation, polymerized conjugated diolefin monomer units and the arms having a number average molecular weight within the range of 3,000 to 15,000.
None of the polymer references listed above disclose lubricating compositions that simultaneously achieve acceptable viscosity index (VI), oil blend thickening capabilities, improved fuel economy, good shear stability, good low temperature viscosity performance, and low viscosity modifier treatment level whilst maintaining the appropriate lubricating antiwear performance for a mechanical device.
Thus there is a need for antiwear agent capable of providing acceptable wear performance over the general lubricant change cycle, whilst not having a detrimental impact on at least one of viscosity index (VI), oil blend thickening capabilities, fuel economy, shear stability, low temperature viscosity performance, and viscosity modifier treatment level. In addition it would be more desirable if the antiwear agent also has no detrimental impact on filter plugging, poisoning of catalyst or other after treatment devices, pollution levels, or accumulation of sludge and/or sediment. The present invention provides a solution to the problem highlighted above.